I think Mick J's points are interesting.

The fuel pumps were replaced. Assuming this was the result of intelligent diagnosis, you may indeed have a faulty fuel pump relay. When fuel pumps get old, they often times produce excess amperage which can damage/destroy a fuel pump relay.

Jumpering the pins as Mick suggested could answer some questions.

There have been many write ups here regarding this process and there may even be some visual info as well in a previous post(s). Try the SEARCH facility for help in this area.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"